The present invention relates to a method of producing an optical video disc, in particular to a method in which two discs are bonded together so as to produce a two-disc laminated optical video disc or digital video disc.
A conventional method for producing a two-disc laminated optical video disc may be described as follows.
At first, a stamper formed with numerous information pits is installed in an injection moulding machine, then a predetermined amount of a translucent synthetic resin such as polycarbonate resin or acryl resin is injected into the injection moulding machine. Afterwards, by means of the stamper installed in the injection moulding machine, a disc substrate made of a translucent synthetic resin and having information pits transcribed from stamper, may be produced by virtue of the stamper installed in the injection moulding machine.
Subsequently, with the use of a method called sputterring or a method called vacuum deposition, a reflecting film of aluminium or aluminium alloy is formed on one side (having information pits formed thereon) of the disc substrate. Further, with the use of a method called spin coating, the surface of the reflecting film is coated with an ultraviolet-setting resin and is then irradiated with an ultraviolet light, so as to form a protection film thereon. In this way, an optical disc is thus completed.
Finally, with the use of a roll-coater, an adhesive agent of hot-melting type (hot-melt adhesive) is applied to the surface of the protection film of each disc, then two optical discs each produced in the above-discussed manner are bonded together at their protecting films, thus producing a two-disc laminated optical disc.
Recently, a digital video disc which is compact in size and capable of recording information with high density, is produced also by bonding together two optical discs, using the same method as described above.
On the other hand, in a conventional process for producing a two-disc laminated optical disc having information pits formed on only one disc, one disc containing information pits is produced in one injection molding machine, another disc containing dummy signals is produced in another injection molding machine, then the two discs are bonded together so as to form a two-disc laminated optical disc. Alternatively, a disc containing information pits and a disc containing dummy signals are both produced in an identical injection molding machine but using different stampers, then the two discs are bonded together so as to form a two-disc laminated optical disc.
But, there is at least one problem with the above-described process. Namely, since it is necessary either to employ two different injection molding machines, or to have a time interval between the productions of the two discs if only one injection molding machine is employed but using successively two stampers, there will be a difference in moisture content between the two discs made of synthetic resin. Hence, after the two discs are bonded together, it is apt for the final product (a two-disc laminated optical disc) to suffer from a problem called deformation or warpage.
Particularly, in a case where a digital video disc is manufactured, since a disc substrate has only a thickness of 0.6 mm which results in a week rigidity, it will be further apt for a final disc product to produce a deformation or warpage due to a difference in moisture content between the two discs made of synthetic resin. In order to overcome such a problem, it has been suggested that every two discs are aged for 24 hours, this however is not preferable since productivity will become lower.